The present invention relates to a method of forming a projection (or projections) on a metal member by means of a new kind of the friction stir processing and a projection-forming device, wherein the projection is of the same material as the metal member; and relates to a metal part that is processed by the method of forming a projection.
A technique of forming a solid projection (a boss) on the surface of a metal member has been widely used as a means for positioning, supporting, fixing or securing, or cooling. The technique is now an indispensable technology in, such as, mobile information terminals like mobile telephones and mobile personal computers, home appliances, automotive parts, and railway cars. A known and currently in-use method of forming a solid projection on the surface of a metal member includes a method such that a projection-part provided in advance is welded, blazed, solid-phase welded, or friction bonded onto a metal member; and a method in which the metal member is pressed to form a projection using a die assembly. Both the techniques of welding and brazing have problems in that they are costly because they needs increased number of steps in processing and in that almost all of these techniques are not applicable to metals such as magnesium, magnesium alloys, aluminum, or aluminum alloys which have been increasingly used for weight reduction purpose of devices. There are another problems in that friction welding does not provide a good dimensional accuracy and in that pressing is not applicable to the cases where the metal member is small in size, thin in thickness, or complicated in structure.
A technique proposed as a method of forming a solid projection on the surface of a metal member is called as a friction stir forming method (Patent Literature 1). In the proposed method, a first object of an aluminum plate is placed on a second object having a recess (a die assembly); a frictional-heat-generation-stirring-means is abutted against the first object; a relative motion is given between them to generate frictional heat to cause the first object to flow plastically under non-molten state; and a shape of projection that corresponds to the recessed portion on the second object is copied on the first object (so-called “coining”). One of other methods proposed for the same purpose is defined in a method for manufacturing magnesium alloy parts (Patent Literature 2). In the method, a plate material is sandwiched between a pair of upper and lower molds and heated to cause a part of the plate material to flow plastically into a recess on the mold to form a projection. The former method, a friction stir forming method, can solve above-stated problems. This method does not need any heating sources, wherein the recess formed on the mold arranged opposite to the stirring means is filled by plastic flow caused by frictional heat. This (the former) method however involves a problem left unsolved in that projections formed with this method are poor in mechanical properties (strength and ductility) with a little practicality. This is because of the fact, according to an investigation by the inventors of the present invention, that the structure in the projection portion is not refined since the recess provided on the second object is positioned apart from such a region where the friction stirring given by the frictional heat generation stirring means on the first object makes the structure fine-grained. Further, it is difficult to form projections having diameters of 3 mm or more with heights of 5 mm or more on a thin plate (2 mm or thinner) with this method. The later method enables forming a projection on a magnesium alloy member that does not accept welding or brazing; the method can solve the above-stated problems. In this method, however, the upper mold and the lower mold severally have a built-in heater for heating the alloy to cause plastic flow. This means that the method has problems in that the thermal energy from outside is required, and besides, the mechanical properties of the projection are not improved. The reason for the poor properties is that a plastic flow occurs without stirring produces only a small strain energy which is insufficient to cause dynamic recrystallization, resulting in an inadequate grain refinement of the structure.
[Patent Literature 1]
Publication of Japanese Unexamined Patent Application, No. 2002-256453
[Patent Literature 2]
Publication of Japanese Unexamined Patent Application, No. 2004-337935